Preparation of polytetramethylene adipamide

Abstract

 A process for the preparation of white nylon 4,6 polyamide of high molecular weight from a tetramethylenediamine adipic acid prepolymer, formed under controlled conditions of temperature and water vapor pressure and wherein the plurality of cyclical end-groups in the prepolymer does not exceed 0.20 mg. equivalent per gram of prepolymer.

Description

[0001] The invention relates to a method for the preparation of polytetramethylene adipamide by heating the salt of 1.4. diaminobutane and adipic acid at elevated pressure till a prepolymer is formed and by further allowing this prepolymer to be condensed to polytetramethylene adipamide with a sufficiently high molecular weight.

[0002] The purpose of the invention is to find a method in the application of which a prepolymer is formed, in a short time, in a liquid phase, which prepolymer can be converted, by aftercondensation, into a white polyamide with a high molecular weight.

[0003] According to the invention a polyamide substantially consisting of units of [NH-(CH₂)₄-NH-C0(CH₂)₄-C0] is prepared by heating the salt of 1.4. diaminobutane and adipic acid and, if desired, other polyamide- forming compounds until a prepolymer is formed, upon which in a following step the prepolymer is converted by aftercondensation at elevated temperature into a polytetramethylene adipamide with a high molecular weight, in which process the prepolymerization is effected by heating the salt to a temperature of between 150 °C and 310 °C and, if desired, keeping the reaction mixture at a temperature of between 150 °C and 310 °C for a limited period, in which process the water vapour partial pressure does not exceed 30 bar and the temperature, pressure and reaction time is chosen so that the amount of cyclical end-groups in the prepolymer thus obtained does not exceed 0,20 mg equivalent per gram of prepolymer.

[0004] With this method white prepolymers can be prepared which can be converted, without problems, by further condensation into white polyamide with a high molecular weight.

[0005] It has been found by the applicant that the prepolymerization of 1.4. diaminobutane with adipic acid is sensitive to side reactions. Presumably, in effecting the reaction under conditions which are not carefully controlled, cyclic derivatives of 1.4. diaminobutane and cyclized oligomers and prepolymers with cyclical end-groups are formed.

[0006] Important for the proper progress of the reaction are the reaction temperature, the temperature at which the reaction mass may be maintained for a limited amount of time, the total reaction time and the water vapour partial pressure at which the reaction is effected. It is also desirable to avoid local overheating of the reaction mass, especially if the reaction mass should be in the solid state for some period. Thus it may be advisable to limit the rate at which the reaction mass is heated in the range between 150 °C and 310 °C to at most 3,5 °C/minute.

[0007] With proper precautions against local overheating and when the reaction mass is liquid, a higher rate could be permitted. For practical considerations a heating rate of at least 0.1 °C/minute will often be applied. A heating rate of between 0.5 °C/minute and 2 °C/minute will be suitable in any case. The rate at which the reaction mass is heated to 150 °C is less critical. The temperature can be increased in a very short time to 75 to 100 °C.

[0008] The salt of 1.4. diaminobutane and adipic acid can be supplied in solid and dry form or as a moist mass or a solution or slurry in water. Preferably, the quantity of water supplied is 50 % by weight at most calculated in respect of the quantity of polyamide-forming compounds and water. When using substantial quantities of water, the water vapour partial pressure can, at a temperature increase, be kept below the 30-bar limit by blowing off steam. In connection with the aftercondensation an excess of 1.4. diaminobutane is preferably applied, as further specified in the patent application (DSM code 3167) filed simultaneously.

[0009] The reaction mixture is heated to a temperature of between 150 °C and 310 °C. At a temperature below 150 °C the molecular weight of the prepolymer is too low. A temperature of more than 310 °C strongly tends to result in the formation of discoloured prepolymers and prepolymers that are more difficult to aftercondense. A temperature of between 175 °C and 300 °C is the most suitable to prepare a white prepolymer with a sufficiently high molecular weight that can be properly aftercondensed. A temperature of between 175 °C and 220 °C is particularly preferred. The reaction mixture can be heated to the desired temperature in one single step, but the mixture can also be heated in two or more steps and, in between, be kept, for a longer or shorter time, at a temperature of between 175 °C and 300 °C. The total duration of the prepolymerization, including heating time, is generally between 0,5 and 6 hours.

[0010] It has been found to be advantageous, if the reaction is carried out at a temperature exceeding 220 °C, to keep the reaction time in the range between 220 °C and 310 °C to the minimum necessary to heat the reaction mass to the required temperature.

[0011] In the temperature range between 150 °C and 210 °C, and preferably 175 °C to 210 °C, the reaction mass may be kept at a constant temperature for longer periods of time, for example during a period of between 15 and 120 minutes.

[0012] Whether or not be the reaction mass is in the solid state or in the liquid state depends on the temperature, the degree of polymerisation and the amount of water in the reaction mass, the last factor being related to the water vapour partial pressure in the reactor. At any stage during the prepolymerisation, the reaction mass may be entirely in the solid state or in the form of a slurry containing solid particles. In order to remove the prepolymer from the reactor, it is preferred that the reaction mass be liquid at the end of the reaction, i.e. at least prior to discharge from the reactor. However, it is preferred to maintain the reaction mass in liquid form during the whole of the prepolymerisation process in order to increase the rate of polymerization, to obtain a homogeneous product and to avoid discolouration by local overheating. This purpose is reached by choosing the water vapour partial pressure (_PH20) so high that sufficient water will remain in the reaction mass to keep the latter in the liquid phase at the temperature at which the condensation takes place. The water is formed in a sufficient quantity in the condensation reaction, and, if desired, water can, in addition to the salt, be fed to the reactor as well. A P_H20 of about 20 to 25 bar, at temperatures of between 150 °C and 300 °C, will in general and depending on the degree of polymerisation suffice for this purpose. At the prepolymerisation temperature, however the P_H20 must not be above 30 bar. At a P_H20 of more than 30 bar a prepolymer is obtained which is difficult to convert into a high molecular polyamide by aftercondensation. Preferably, the P_H20 will be less than 5 bar above the minimum partial pressure necessary to maintain the reaction mass in the liquid state.

[0013] As the prepolymer has a melting point of between about 270 °C and 290 °C, it is possible to obtain the prepolymer in liquid form, virtually free of water, by heating the reaction mass to a temperature exceeding the melting point and releasing the pressure near or at the final temperature.

[0014] The prepolymerisation thus may be effected by heating the salt, with control of the P_H20, with a continuous increase of the temperature to e.g. 290 "C or 300 °C or by heating the salt to a temperature of preferably between 180 °C and 200 °C, maintaining the temperature in said -ange for a limited period and then increasing the temperature to e.g. 290 °C to 300 °C.

[0015] It is also possible to end the prepolymerisation at a temperature below 270 °C by discharging the reaction mass in the liquid state from the reactor and releasing the pressure to atmospheric. The water will then escape from the reaction mass and the prepolymer will be obtained in solid form.

[0016] Applicant has found that it is very important to limit the number of cyclical end-groups in the prepolymer as much as possible. These cyclical end-groups are essentially pyrrolidinyl groups. If there are more than 0.20 mg eq/g of cyclical end-groups, it will be very difficult to convert the prepolymer within a reasonable time to a high molecular weight polyamide.

[0017] Prepolymers with a cyclical end-group content between 0,10 and 0,20 mg eq/g and better yet between 0.10 and 0.15 mg eq/g can be converted to high molecular weight polymers, provided that they contain a rather large excess of 1.4. diaminobutane of say at least 3 moles-X calculated on the stoechiometric amount of diaminobutane. Preferably the reaction is carried out under such conditions that the content of cyclical end-groups in the prepolymer is at the most 0,10 mg eq/g, because in that case the prepolymer can easily be converted to a high molecular weight polymer.

[0018] The salt can be prepared as known in the art, for instance by adding adipic acid to a solution of 1.4. diaminobutane until the equivalence point has been reached. If desired, extra diamine can be added to the salt. As specified in the patent application (DSM 3167) filed simultaneously, there are advantages in applying an excess of diamine of between 0.5 and 15 moles X, more specifically between 1.5 and 5 moles-X. An excess of diamine does not adversely affect the prepolymerisation. The salt can be fed to the prepolymerisation reactor in dry form or as a moist mass or as a solution or slurry in water. As reactor, an autoclave or pressure reactor, whether or not provided with a stirrer, can be used. The reaction can also be effected continuously in a reactor suitable for this purpose. It is recommended to effect the prepolymerisation in the absence of oxygen in order to prevent discolouration, and to use a reactor material which is resistant to corrosion by the hot alkaline reaction mass, e.g. titanium.

[0019] The prepolymer is subsequently subjected to aftercondensation. The aftercondensation is preferably effected in the solid phase at a temperature of between 200 °C and 275 °C, more specifically between 225 °C and 275 °C, in an atmosphere containing water vapour.

[0020] The aftercondensation is described in more detail in the patent application (DSM 3167) filed simultaneously.

[0021] The reaction mixture, which is subjected to the prepolymerisation, may contain additives customary for polyamides, such as stabilizers, antioxidants, matting agents, etc.

[0022] In addition to diaminobutane and adipic acid, up to 20 % by weight of other polyamide-forming compounds can be worked up in the prepolymer. In this connection caprolactam, undecalactam, laurinolactam and other lactams, amino carboxylic acids, such as 11-amino-undeca-carboxylic acid, diamines such as hexamethylene diamine, and dicarboxylic acids such as succinic acid, sebacic acid, oxalic acid isophtalic acid and terephthalic acid, may be though of. Preference, however, is given to the preparation of the homopolyamide, which has very good physical properties.

[0023] The number-average molecular weight of the prepolymer prepared by applying the method according to the invention may generally be between 1000 and 15,000. Preferably the reaction conditions, particularly the temperature, the water content and the duration of the reaction, are chosen so that a prepolymer with a molecular weight of between 2000 and 10,000 is obtained. By aftercondensation a polyamide can be obtained with a molecular weight of between 15.000 and 75.000. These polyamides are very suitable for making yarns and various objects.

[0024] The invention is elucidated by means of the following examples without being restricted to the embodiments described therein.

Example 1

[0025] A. The solid of 1.4. diaminobutane and adipic acid was prepared by dissolving 1 part by weight of the diamine in 8 parts by weight of methanol and putting this solution in a reactor provided with a stirrer and reflux cooler. Subsequently, while stirring, solid adipic acid was added slowly until the equivalence point was reached. After cooling, the precipitated salt was filtered off, washed with cold methanol and dried. Then 1.4. diaminobutane was added to an excess of 2.1 moles-X. In the following examples the salt was prepared in the same way, unless specifically stated otherwise.

[0026] B. A 5-litre autoclave was filled with 1700 g of the salt obtained underA and 170 of water. After flushing with nitrogen the mass was heated, in 80 minutes, to 180 °C and kept for 100 minutes at that temperature. In this process the pressure increased to, and was maintained at, 8 bar. After that the pressure was reduced to 1 bar, and the reaction mass was allowed to cool. The solid white prepolymer was taken from the reactor. It had a relative viscosity of 1.33 (1 g of polymer in 100 ml 96 % sulfuric acid at 20 °C) and a cyclical end group content [pyr] of 0,032 mg eq/gram polymer.

[0027] In the examples following hereafter the same method was applied for the prepolymerisation, albeit with different temperatures, times and pressures. With the use of a salt not containing an excess of diaminobutane the same result was achieved.

[0028] C. The prepolymer was ground to a particle size between about 0.1 and 0.5 mm and put in a rotating reactor suitable for effecting reactions. After flushing with nitrogen, an oxygen-free mixture of 30 % by volume of steam and 70 % by volume of nitrogen, heated to 260 °C, was passed, at a pressure of 1 bar, through the rotating reactor. The prepolymer was subjected in this way to aftercondensation for 6 hours at a temperature of 260 °C, the gas mixture specified above being passed through.

[0029] This method has been applied also in the following examples for the aftercondensation.

[0030] Thus a white polytetramethylene adipamide was obtained with an η_rel of 4,65 and [pyr] of 0,005 meq/g.

Example II

[0031] In the way described in example I a prepolymer was prepared by heating the salt, which contained an excess of 8 moles % of diaminobutane, in 55 minutes from 75 °C tot 155 °C and, successively, in 160 minutes from 155 °C to 200 °C. The pressure was 19 bar at most.

[0032] The prepolymer thus obtained was white and had an η_rel of 1,23 (M_n of 3500) and [pyr] of 0,028.

[0033] By aftercondensation (6 hours at 260 °C) a white high molecular polyamide could be obtained with a η_rel of 4,90 corresponding to a M_U of 34,700.

Example III

[0034] A mixture of 15 parts by weight of water and 100 parts by weight of 1.4. diaminobutane-adipic acid salt, which contained an excess of 5.4 moles-% of diamine, was heated in 75 minutes from 75 °C to 175 °C and, successively, heated in 100 minutes to 207 °C. The pressure was limited, by blowing off, to 10 bar. The prepolymer had an η_rel of 1,20 (M_n of 4500) and [pyr] of 0,122. By aftercondensing for 6 hours at 260 °C, it was converted into a white polyamide with an η_rel of 3,37, corresponding to an M_n of 27,400.

Example IV

[0035] A mixture of 15 parts by weight of water and 100 parts by weight of 1.4 diaminobutane-adipic acid salt, which contained an excess of 8 moles-X of diamine, was heated in 240 minutes from 20 °C to 210 °C. The pressure increased to 19 bar. The prepolymer could be converted by aftercondensation (6 hours at 260 °C) into a white polyamide with an M_n of 24,800 (η_rel of 2,85).

Example V

[0036] The salt of 1.4. diaminobutane and adipic acid was obtained by adding a solution of the dicarboxylic acid in methanol to a solution of the diamine in methanol until the equivalence point was reached. The reaction mixture was cooled and the salt was recovered by filtration, subsequently washed with methanol and dried. To the dry salt 1.3 % by weight of the diamine, calculated in respect of the salt, was added.

[0037] The salt was put in an autoclave. After flushing the autoclave was heated. In 220 minutes a temperature of 215 °C was reached, heating rate 0.8 °C/minute on average, during the experiment varying between 0.15 °C/minute and 2.3 °C/minute. In this process the pressure increased from 1 bar to 14.6 bar. This was followed by blowing off to a pressure of 1 bar, and the reaction mass was cooled. Thus a solid white prepolymer with an M_n of 3500 was obtained (η_rel of 1,38).

Example VI

[0038] In the way described in example V a salt was prepared with an excess of 1.3 % by weight of 1.4. diaminobutane.

[0039] The prepolymerisation was effected by heating the salt in an autoclave to 233 °C in 245 minutes at an average heating rate of 0.8 °C/minute. In this process the pressure increased to 18.7 bar. The prepolymerisation was discontinued by reducing the pressure to 1 bar aud cooling the reaction mass. Thus a white prepolymer with an M_n of 4100 was obtained (η_rel of 1,44).

Example VII

[0040] A mixture of 1700 g of the salt of adipic acid and 1.4. diaminobutane, containing 2,4 mol.% excess diaminobutane, and 170 g water was heated in 60 minutes to 180 °C and then in 50 minutes from 180 °C to 295 °C. In the temperature range of 200 °C to 260 °C the pressure was limited by venting to 13 bar and in the range of 260 °C to 295 °C pressure was gradually released so as to reach 1 bar at 295 °C. The molten prepolymer was discharged from the reactor, cooled and ground. The white prepolymer had an η_rel of 1,66 and [pyr] of 0,064 meq/g. It was converted by solid phase aftercondensation (4 hrs at 260 °C) to a polyamide of η_rel of 2,65. The same aftercondensation conditions were used in the following examples unless stated otherwise.

Example VIII

[0041] The procedure of example VII was repeated using the same starting composition, with the difference that the temperature was increased in 60 minutes to 180 °C, kept at this value for a further 60 minutes with a pressure of 8 bar, after which the reaction mass was heated to 295 °C in 40 minutes. The prepolymer had an η_rel of 1,66 and [pyr] of 0,053 meq/g and could be converted to a polyamide of n_rel 2,75.

Example IX

[0042] The same starting mixture as in example VII was heated in 75 minutes to 220 °C, kept at 220 °C for 20 minutes with a pressure increasing to 18,5 bar. Then the reaction mass was heated to 295 °C in 30 minutes with slow release of pressure starting at 260 °C. The prepolymer had an η_rel of 1,63 and [pyr] of 0,078 meq/g. After condensation (again 4 hrs at 260 °C) yielded a polyamide of η_rel 2,65.

Example X

[0043] The procedure of example IX was followed with the sole difference that the reaction mass was kept at 220 °C for 120 minutes, with a pressure increasing to 22,5 bar. The prepolymer had an η_rel of 1,49 and [pyr] of 0,192 meq/g. Aftercondensation yielded a polyamide of n_rel 1,81. This shows that it is not advisable to maintain a temperature above 200 °C for too long a time, as this leads to a rise of [pyr] and a lower molecular weight of the end product.

Example XI

[0044] The procedure of example IX was followed, but now the temperature was increased to 250 °C in 80 minutes, kept at 250 °C for 20 minutes with a pressure increasing to 27 bar, and then increased to 295 °C in 20 minutes. The prepolymer had an η_rel of 1,49 and [pyr] of 0,173 meq/g. Aftercondensation yielded a polyamide of η_rel 1,90.

Example XII

[0045] The procedure of example VIII was repeated but now the molten prepolymer was kept at 295 °C and 1 bar (steam) for 60 minutes. The resulting prepolymer had an η_rel of 1,73 and [pyr] of 0,072 meq/g. After condensation yielded a discolored polyamide of η_rel 2,38.

Example XIII

[0046] The procedure of example VIII was repeated but using this time a mixture of 1000 g of the salt and 300 g of water. A prepolymer of η_rel 1,64 and [p^yr] of 0,074 was obtained which could be converted (4 hrs at 260 °C) to a polyamide of η_rel 2,50.

Example XIV

[0047] A mixture of 1000 g of butanediamine-adipic acid salt (diamine excess of 2,4 mole %) and 700 g of water was heated to 180 °C in 60 minutes, kept at 180 °C for a further 60 minutes with the pressure maintained at a maximum of 6 bar, and the reaction mass was then heated to 295 °C in 35 minutes with a maximum pressure of 13 bar and slow pressure release starting at 260 °C. The prepolymer had an n_rel of 1,65 and [^pyr] of 0,068 meq/g. Aftercondensation (4 hrs at 260 °C) yielded a polyamide of η_rel 2,47.

Example XV Not according to the invention

[0048] A procedure which is suitable for the production of nylon 6.6 was followed, starting with a mixture of 1000 g of the diaminobutane-adipic acid salt (diamine excess 2,4 mole %) and 700 g of water. The mixture was heated to 215 °C in 60 minutes and then in 120 minutes from 215 °C to 275 °C, the pressure being kept at a maximum of 18 bar. Then the reaction mass was heated to 295 °C in 15 minutes with slow release of pressure. The white prepolymer had an η_rel of 1,36 and [pyr] of 0,263 meq/g. Aftercondensation (4 hrs at 260 °C) yielded a polyamide of _nrel 1,71. This procedure is clearly unsuitable for the production of nylon 4.6.

Example XVI

[0049] The procedure of example XV was repeated with the difference that the temperature was increased from 215 °C to 275 °C in 30 minutes. The prepolymer now had a η_rel of 1,50 and [pyr] of 0,143. By aftercondensation the η_rel was increased to 1,88.

Example XVII

[0050] A mixture of 100 parts by weight of the diaminobutane-adipic acid salt (diamine excess 2,0 mole X), 10 parts of caprolactam and 10 parts of water was heated to 180 °C in 60 minutes and kept at 180 °C for 180 minutes with the pressure being kept at a maximum of 8 bar. The prepolymer was then obtained as in example I.

[0051] It had an η_rel of 1,18 and [pyr] of 0,046 meq/g. Aftercondensation (4 hrs at 260 °C) yielded a nylon-6.46 copolymer of η_rel 3,25.

Example XVIII

[0052] The procedure of example XVII was repeated but now using 10 pbw of 11-amino undecanoic acid. The prepolymer had an η_rel of 1,20 and [pyr] of 0,030 meq/g. Aftercondensation (4 hrs at 260 °C) yielded a nylon 11.46 of η_rel 2,51.

[0053] When the example was repeated using 20 pbw of 11-amino undecanoic acid, the resulting prepolymer had an q_rel of 1,18 and [pyr] of 0,035, aftercondensation (6 hrs at 250 °C) yielding a copolyamide of η_rel 2,42.

Example XIX

[0054] A number of prepolymers were prepared, starting from a salt of 1.4.diaminobutane and adipic acid, containing 1,1 mole % excess diaminobutane, and varying amounts of water. The mixture in each case was heated in about two hours to a given temperature, kept at said temperature for 5 minutes, after which the reaction mass was discharged from the reactor and cooled with release of pressure. The relative viscosity and content of cyclical end-groups of each prepolymer were determined. The prepolymers were then subjected to aftercondensation for 4 hours at 260 °C as in example I.

[0055] In table 2 the water content of the starting mixture, the maximum temperature and maximum pressure and the physical data of the prepolymer and the final polyamide are given.

Determination of the pyrrolidine endgroups in nylon 4.6

[0056] A glass tube containing 0,25 g of dry polyamide and 0,5 ml of 6N hydrochloric acid is flushed with nitrogen, sealed and heated to 130 °C in an oil-bath. This temperature is maintained until the polyamide dissolves and for four hours afterwards. The tube is then cooled, with attendant formation of crystals of adipic acid, and opened.

[0057] A sample for gas-chromatographic analysis is obtained by diluting 0,20 ml of the supernatant liquid with 0,70 ml of 2N alcoholic sodium hydroxide. Analysis is carried out by bringing 1 µl of the sample on a preconditioned chromosorb (RTM) 130 column of 1,5 m x ¼" which is heated for one minute at 150 °C and then heated to 220 °C with a heating rate of 13 °C/minute.

[0058] Detection is carried out by catharometer method.

[0059] The pyrrolidone peak is detected after 4,5 minutes and the 1.4.diaminobutane peak after 8 minutes. The diaminobutane content of nylon 4.6 is 0,005 mole/g.

[0060] The pyrrolidone content is calculated on the base of the ratio between the area of the diamino-butane peak and the area of the pyrrolidone peak with the formula [pyr] - 5000/peak ratio, expressed in mmol pyrrolidine/g of polymer.

Claims

1. Method for the preparation of a polyamide substantially consisting of units of [NH-(CH₂)₄-NH-CO-(CH₂)₄-CO] by heating the salt of 1.4. diaminobutane and adipic acid and, if desired, other polyamideforming compounds, with formation of a prepolymer, followed by further condensation to a polyamide with a high molecular weight, characterized in that the prepolymerization is effected by heating the salt to a temperature of between 150 °C and 310 °C and, if desired, keeping the reaction mixture at a temperature of between 150 °C and 310 °C during a limited period, in which process the water vapour partial pressure does not exceed 30 bar, and the combination of temperature, pressure and reaction time is chosen so that the amount of cyclical end-groups in the prepolymer thus obtained does not exceed 0,20 mg equivalent per gram of prepolymer.

2. Method according to claim 1, in which the reaction temperature is between 175 °C and 300 °C.

3. Method according to claims 1-2, in which the content of cyclical end- groups in the prepolymer does not exceed 0,10 mg eq/g.

4. Method according to claims 1-3, characterized in that the reaction period in the temperature range of between 220 °C and 300 °C is kept to a minimum.

5. Method according to claims 1-4, characterized, in that the temperature and water vapour partial pressure in the final stage of the prepolymerization are chosen so that the reaction mass can be discharged from the reactor in liquid form.

6. Method according to claims 1-5, characterized in that the water vapour partial pressure is chosen so that the reaction mass is in the liquid state during the whole prepolymerisation.

7. Method according to claims 1-6, characterized in that the water vapour partial pressure is not more than 5 bar higher than the minimum partial pressure necessary to keep the reaction mass in molten form at a given temperature and degree of polymerisation.

8. Method according to claims 1-7, characterized in that the temperature is between 175 °C and 220 °C.

9. Method according to claims 1-8, characterized in that a mixture of the 1.4. diaminobutane-adipic acid salt with 50 % by weight of water at most, calculated in respect of the quantity of salt and water, is started from.

10. Method according to claims 1-9, characterized in that a prepolymer is prepared with a number-average molecular weight of'between 1000 and 15,000.

11. Method according to claims 1-10, characterized in that a prepolymer is prepared with a number-average molecular weight of between 2000 and 10,000.

12. White polytetramethylene adipamide, prepared by applying the method according to any one or more of the claims 1-11.